In my last segment in Kibbles and Bytes I did my best to cram in as much introductory information as I could about alternating current. I talked about the difference between AC and DC and why we use AC at the electricity grid level. For the most part though I didn’t get into some of the finer details about AC, other than that it’s current that alternates and this makes it very easy to transform the voltage of AC power.
One of the most difficult things to wrap your head around when it comes to AC is the fact that at any given time, the voltage is different. If you charted AC voltage on a graph with time on the x-axis, you’d see a sine wave.
All waves have some general properties such as amplitude, frequency and phase. Amplitude and frequency you’re probably familiar with already from radio, and in the case of AC they mean the same thing. In the US, the frequency of AC power is 60Hz. This means that it cycles up and down 60 times every second. In other countries around the world the frequency is 50Hz. Actually, only North America, large parts of South America, Saudi Arabia, the southern part of Japan, North and South Korea and Taiwan use 60Hz. Generally speaking, the frequency of the AC power won’t negatively affect electronics as long as the voltage is the same. Most of the areas that use 50Hz also use voltages in the 220-240 range though. Sometimes the 60Hz frequency is used to drive clocks in electronics, though with digital circuitry, this is less common today. So if you had a piece of equipment with a clock that was driven by 60Hz AC, using 50Hz AC would prevent the clock from being accurate.
Amplitude of AC is where things become a little less straightforward. In North America, standard household power is 120VAC. You might think this means that the AC signal oscillates between +120VAC and -120VAC, but this is not the case. Actually, a 120VAC signal will oscillate between +169.7VAC and -169.7VAC. Why do we call it 120VAC power then? Well, think about the AC wave. At any given time its value is different. When describing the power, what value should you use? 169.7? Why not 40, or even 0? As you can see, choosing any voltage on the AC waveform would be arbitrary and not representative of the power being delivered. 120VAC is the RMS (root mean square) of the AC signal. The math behind this starts to get complicated, but at least in the context of electricity, the RMS of the AC signal is a representation of the average power delivered by that signal. In other words, the amount of power delivered by a 120VAC signal is equivalent to 120V of direct current.
Do you wish that was the end of the story? It’s not. When we talk about power delivered in this case, we have to talk about power delivered into a purely resistive load. This is because once again, AC is a sine wave. Now we arrive at the third property of a wave: its phase. The phase can be described as when the signal passes through 0 to go from negative to positive or positive to negative. In AC power, phase is very important because certain types of loads (capacitive or inductive) can shift the phase of components of the signal. Normally we would think about an AC signal as two things in one: voltage and current. The voltage goes up and the current goes up with it proportionally in phase. This is not always the case however. Using inductive or capacitive loading, you can cause the phase of parts of the signal to shift so that, for example, the voltage leads the current by half a cycle.
Let’s try to round this out. In AC circuits, the concept of direct resistance doesn’t really apply the same way that it does in DC circuits. This is, again, because the voltage in AC is constantly changing. A changing current will create a changing magnetic field, and based on the properties of Lenz’s Law, a magnetic field will be proportional to the electrical current that created it, but it will be in direct opposition to it. In a way, it’s analogous to Newton’s third law of motion. These generated magnetic fields can provide a resistance to changing current via certain devices like inductors. This causes the current component of the signal to advance its phase ahead of the voltage component in the case of inductive loads and behind the voltage component in the case of capacitive loads. This “resistance” in AC circuits is called impedance and the combination of inductive and capacitive factors in a circuit forms the imaginary concept of reactance.
Don’t feel bad if all of this seems very confusing. When I took some electrical engineering classes in college (not my major, I took them for fun), they were the only classes I really struggled with. I’ve actually learned more and understood things better by watching videos on YouTube and reading books. I also learned a lot in designing and assembling my photovoltaic power station. I think anybody can learn this stuff, and it’s very rewarding when you do.